The invention relates to a cooling process for recovering and/or fractionating a mixture composed mainly of butane and propane, generally designated by the name NGL ("Natural Gas Liquid"), present in a crude gas, such as a gas associated with petroleum, a natural gas or condensate gas, which process uses an external mechanical loop through which a cooling fluid passes.
Apart from butane and propane, the NGL mixture may also contain lighter components, such as ethane, and heavier components, such as pentane and even hexane. Fractionation of the NGL mixture produces petroleum gases known by the name LPG ("Liquefied Petroleum Gases"), such as commercial propane and commercial butane.
Hitherto, the recovery and fractionation of an NGL mixture present in a crude gas, with an external mechanical cooling loop, has been carried out using a pure cooling fluid, such as, for example, Freon 12, ammonia or propane. Loops of this type, which are described, for example, in the 1972 edition of the publication of the NGPSA ("Natural Gas Processors Suppliers Association"), section 5, FIGS. 5-14 and 5-15, and in "World Oil" of September 1961, pages 83 to 96, have the disadvantage of limiting the possibilities of recovery or fractionation of the gases to be treated, because of the very nature of the cooling fluid used; for example, propane does not make it possible to fall below -35.degree. C. These loops thus lack flexibility and are difficult to adapt as soon as the actual conditions of the gas to be treated tend to deviate from the base conditions.
Admittedly, it is possible to increase the amount of NGL mixture produced, by installing a second cooling loop through which a more volatile cooling fluid than the first passes, and which makes it possible to produce a further lowering of the temperature of the gas to be treated. However, this results in an expensive proliferation of the equipment installed.
Furthermore, in the case where the plant for recovering and/or fractionating the NGL mixture is located in a desert region or a region which is a long way from any source of supply of pure cooling fluid, it can result in problems regarding the supply of coolant and hence in a reduction, or even a halt, in the production of the plant.
One method providing a possible alternative to the use of an external mechanical cooling loop consists in installing an expansion turbine acting on the gas from which it is desired to extract the NGL mixture or the LPGs. However, this method is only industrially practicable if the pressure of the gas to be treated is sufficiently high and if the quality of the gas to be treated undergoes little or no variation throughout the operating life of the plant. If this is not the case, it is necessary to add an external mechanical cooling loop.
The invention relates to a cooling process using an external mechanical loop through which a cooling fluid passes, for recovering and/or fractionating an NGL mixture present in a crude gas, which provides this loop with a high degree of flexibility of use, both in the range of temperatures which can be reached and in the independence of its operation in relation to external supply sources.
According to the invention the mixture itself, composed mainly of butane and propane, referred to as NGL, extracted from a crude gas, forms the cooling fluid of the external mechanical loop which is required for the production and/or fractionation of this NGL mixture.
As stated above, the NGL mixture produced from a crude gas contains ethane, propane, butane and a few slightly heavier hydrocarbons, such as pentane and even hexane. This ethane content ensures that the b.p., at atmospheric pressure, of the NGL mixture is generally lower than that of the cooling fluids ordinarily used (for example pure propane). This has the main advantage of permitting a greater lowering of the temperature of the gases to be treated.
Furthermore, the mechanical cooling loop through which a fluid of this type passes does not require an auxiliary supply, outside of the plant for production of NGL and/or LPG, in order to compensate the losses during normal operation of the plant. If, as is generally the case, the plant is equipped with a stock of NGL mixture, an appropriate cooling fluid is then immediately available for bringing the plant back on stream after a normal or unforeseen shutdown. Only the first start-up of a plant for the production of NGL requires an external supply either of NGL mixture of a quality similar to that of the NGL mixture to be produced, or of propane to which some crude gas is added, provided that it is treated in order to remove the moisture and the corrosive components therefrom.
The additional flexibility in the operation of a plant for the production of NGL mixture should also be noted, this flexibility being obtained by the use of a cooling fluid of this type in an external mechanical cooling loop; in fact, the degree of recovery, from the crude gas, of the NGL mixture produced by the plant is directly related to the temperature obtained with the aid of the cooling loop; now, with the process according to the invention, this temperature is a characteristic of the quality of the NGL mixture produced and hence of the recovery.
The process according to the invention makes it possible to improve the performances of an existing plant for recovering the NGL mixture, of which it is desired to increase production by recovering the additional NGL mixture still present in the already treated gas leaving the plant. Since the existing external mechanical cooling loop does not permit this increase in production, as a result of the low-temperature limitation of the pure cooling fluid which passes through this existing loop, it is necessary to install a second mechanical loop which uses a more volatile, pure cooling fluid than the one already in use. Apart from exceptional cases, where it will be possible to pass the NGL mixture produced through the existing loop, the invention will not dispense with the installation of this second loop, but it will provide the latter with significant advantages by passing through it the NGL mixture produced by the plant, instead of a pure cooling fluid. On the one hand, the cooling fluid of the second loop is thus immediately available on site, without it being necessary to obtain an external supply of a pure cooling fluid, such as, for example, ethane. On the other hand, the fluid used to condense the NGL mixture in the second mechanical loop will be the same as that used to condense the pure cooling fluid in the existing loop, that is to say air or water. Interdependence between the two loops of a cascade of the propane-ethane type, for example, in which the ethane must be condensed with propane, therefore no longer exists. This results in a reduction in the installation costs, the financing expenses and also the size of the equipment to be installed. If appropriate off-takes are provided on the heat exchangers for the gas to be treated, the two cooling loops can furthermore be used to back each other up.
The invention also applies to the production of liquefied natural gas (LNG) with the use of several external mechanical cooling loops (propane loop, ethane or ethylene loop and methane loop, in the case of a conventional so-called cascade system). It will be possible, for example, to replace the propane loop, or both the propane loop and the ethane or ethylene loop, by an NGL loop according to the invention.